In electrophotographic machines a photoconductive material is charged in a uniform manner and then exposed to light reflected from an original to be copied. That light variably discharges the photoconductive material thus providing an image of the original on the photoconductive surface. The image is then developed through the application of toner which typically is a black powdery substance electrically attracted to the undischarged areas of the image. After development the image is transferred to a piece of copy paper and fused thereto.
Magnetic-brush developers provide high quality development of latent images. These developers generally comprise a rotating hollow shell made of non-magnetic material with numerous stationary magnets disposed within. A developing mix is typically comprised of small steel beads, called carrier, which are magnetically attracted to the surface of the rotating roll by virtue of the magnets disposed therein and the above-mentioned toner. The small steel beads are coated with the black powdery toner and carry the toner along the surface of the magnetic-brush roll into the development zone where the toner may be electrically attracted from the steel beads to the latent image. A typical magnetic brush developer is shown in U.S. Pat. No. 3,999,514; which patent is incorporated herein by reference. This invention represents an improvement to the mechanism shown in the above-mentioned patent in which the magnetic-brush roll is used to transport the developer mix out of the sump area as well as to deliver the developer mix to the latent image at the development zone.
In magnetic-brush developers it is important to achieve steady and uniform carrier bead flow from a pickup area to the development zone. In that manner, an ample supply of toner will be provided to develop the latent image. However, erratic and unstable carrier bead flow can occur if carrier beads pack or jam while being carried toward the development zone. Such a condition was found to occur at times in the developer of the above-mentioned patent near the top doctoring blade just prior to entering the development zone.
The inventors herein have discovered that this condition can be avoided by providing a non-magnetic overflow plate to separate the transport area of the developer into two separate paths. In the first path, next to the rotating magnetic-brush shell, carrier beads are transported in the customary manner from a pickup zone toward the development zone. However, a second path is provided between the overflow plate and the inner surface of the developer housing so that if any excess magnetic carrier beads are present near the top doctoring blade, they may be returned to the pickup zone through that second path. The inventors also discovered that the magnetic fields supplied to transport carrier beads toward the development zone should be separated from the magnetic field at the development zone so that excess carrier beads can be moved away from the surface of the rotating magnetic-brush roll and out to the overflow plate without the binding effect of magnetic attraction during that movement.
As a result of this improvement the bead flow against the photoconductor has been maintained steady and uniform since jamming of excess quantities of carrier beads near the developmemt zone has been eliminated. Additionally, since a packed condition of carrier beads greatly increased the wear rate of carrier coating and caused a milling of toner particles, these undesirable effects were also eliminated. Other advantages of the invention include the fact that power requirements to drive the developer were decreased; the temperature rise of the developer mix was minimized; and certain critical adjustments and parts, such as doctor blade clearance, roll to photoconductor clearance, magnet strength and mix weight in the developer were made less critical.